The present invention relates to acoustic emission. More particularly, the present invention relates to detecting acoustic emissions using a microwave Doppler radar detector.
1. Acoustic Emission
ASTM E 610-89a (1990) defines acoustic emission (AE) as “the class of phenomena whereby transient elastic waves are generated by the rapid release of energy from a localized source or sources within a material, or the transient elastic waves so generated” (p. 269). Li (2002) states that, “Clearly, an AE is a sound wave or, more properly, a stress wave that travels through a material as the result of some sudden release of strain energy” (p. 157).
Investigators have developed many acoustic emission (AE) instruments and systems for both monitoring and nondestructively testing structures, materials, manufacturing processes, and devices. AE has been used for nondestructively testing refineries, pipe-lines, power generators (nuclear or other), aircraft, offshore oil platforms, paper mills, and structures (bridges, cranes, etc.). AE sensors have also been used for quality control in manufacturing operations and in research applications, involving composite structures such as fiberglass, reinforced plastics, and advanced aerospace materials (Li, 2002).
AE is also generally rated one of the most effective indirect methods for monitoring tool condition in machining operations. The major advantages of using AE to monitor tool condition are that (1) the frequency range of the AE signal is much higher than that of the machine vibrations and environmental noises, and (2) AE measurements do not interrupt cutting operations (Li, 2002).
As shown in FIG. 1, possible sources of AE during metal-cutting processes include:                1. Plastic deformation in the workpiece during the cutting process;        2. Plastic deformation in the chip;        3. Frictional contact between the tool flank face and the workpiece, resulting in flank wear;        4. Frictional contact between the tool rake face and the chip, resulting in crater wear;        5. Collisions between chip and tool;        6. Chip breakage;        7. Tool fracture.        
Based upon analysis of AE signal sources during machining, AE consists of both continuous and transient signals, which have distinctly different characteristics. Continuous signals are associated with shearing in the primary zone and wear on the tool rake and flank faces. Burst or transient signals result from either tool fracture or chip breakage. Friction between workpiece and tool and tool fracture are regarded as the most important sources of continuous and transient AE signals in turning operations (Li, 2002).
2. Early AE Sensors
In 1996, Drouillard provided a comprehensive history of early acoustic emission research. Relevant highlights from his article follow.
In the early 1900s, metalworkers often reported audible sounds (clicking, chatter; squeaks, grinding, hissing, and snapping) emitted by metals, particularly tin and zinc, during twinning and martensitic transformation. Soon thereafter, several investigators conducted instrumented experiments to study the phenomenon (Drouillard, 1996).
From 1925-1929, Klassen-Neklyudova (National Physical-Technical Rontgen Institute in Leningrad, Russia) began a systematic investigation into the very regular cracking noises emitted during plastic deformation of metals. She used an optical method to measure stepwise, jerky movements of a metal specimen during plastic deformation and then correlated the measurements with the cracking noises (Drouillard, 1996).
On Nov. 21, 1933, at a meeting of the Earthquake Research Institute in Tokyo, Professor Fuyuhiko Kishinouye presented the first report on a scientifically planned acoustic emission experiment. Kishinouye studied AE characteristics in wood to understand and develop methods for studying fracture of the earth's crust, which leads to earthquakes. He used a phonograph pick-up with a steel needle to measure stress waves in a wooden board which he bent to fracture. He reported that when the board cracked, an electric current was generated in the coil of the pick-up, which he then amplified and used to drive a seismograph. Kishinouye reported that the instrument recorded both audible and inaudible vibrations during both the bending process and final fracture (Drouillard, 1996).
From 1936-1940, Forster and Scheil (Kaiser-Wilhelm-Institut fur Metallforschung, Stuttgart, Germany) conducted several AE experiments. They developed an electrodynamic transmitter/receiver system to transform mechanical vibrations and acoustic emissions into electrical voltages which could be amplified and recorded. They used the device to measure extremely small voltage changes due to resistance variations produced by sudden, jerky strain movements caused by martensitic transformations in wire-shaped nickel-steel test specimens (Drouillard, 1996).
In 1948, Mason, McSkimin, and Schockley (Bell Telephone Laboratories, Murray Hill, N.J.) reported using a quartz crystal transducer, pressed directly against a tin specimen, to measure acoustic emission. They applied enough stress to deform the specimen and cause twinning dislocations, which, in turn, produced acoustic emission. The quartz crystal they used had a uniform sensitivity from a few kilohertz to 5 MHz (Drouillard, 1996).
In the early 1960s, a special projects team of structural test engineers, led by Allen T. Green (Aerojet-General Corporation, Sacramento, Calif.), used AE measurements to verify the structural integrity of glass-filament-wound Polaris solid rocket motor cases fabricated for the U.S. Navy. They noticed that hydrostatic proof-pressure tests caused audible sounds in the test specimen. The team used microphones, a magnetic tape recorder, and sound-level analysis equipment to detect, record, and then post-test analyze the acoustic signals. Later, they used accelerometers and charge amplifiers to improve their detection capability. In 1965, the team used the method to locate crack initiation and propagation prior to catastrophic failure, at about 56 percent proof pressure, in a steel solid rocket motor case. Their multi-channel analog computer-based system was used to build the first commercial, real-time structural testing system (Drouillard, 1996).
In 1963, Harold Dunegan (Lawrence Livermore National Laboratory, Livermore, Calif.) began a lifelong career in AE, after hearing a paper presentation at the Third Symposium on Physics and Non-Destructive Testing. With co-workers, Dunegan developed practical AE procedures to predict failure in pressure vessels during proof testing, without taking the vessels to failure. He also developed the S140 transducer, which became the workhorse of the AE industry for over 25 years. Dunegan contributed significantly to AE study by developing more effective instrumentation, which eliminated noise due to mechanical vibration of the test specimen. In particular, he developed narrow-banded piezoceramic sensors which operated in frequency ranges well above the audio range (30-150 kHz). (Drouillard, 1996; Dunegan & Harris, 1969).
In the late 1960s, the U.S. Atomic Energy Commission began using AE techniques for nondestructive nuclear reactor testing. They used existing AE sensors, including accelerometers with charge amplifiers and tape recorders, and developed a new high-temperature submersible microphone for use in the liquid sodium environment of a reactor (Drouillard, 1996).
During the late 1960s and early 1970s the Boeing Company (Seattle, Wash.) initiated research on incipient failure detection in bearings, signature analysis from rotating machinery, leak detection in hydraulic systems, and cavitation and erosion detection in fluid flow valves. They also studied crack growth in titanium (Drouillard, 1996).
From 1967-1980, strong AE research helped determine the source of AE and improved methods for detecting crack growth in nuclear reactor pressure vessels and other thick-walled vessels. AE research efforts during the period led to development and commercialization of AE source location instrumentation and software. In 1968, Nortec Corporation (Richland, Wash.) manufactured a plug-in module for the Tektronix oscilloscope. Later that year, Dunegan founded Dunegan Research Corporation (Livermore, Calif.) to supply the first full line of AE sensors and modular instruments. Other U.S. companies also entered the market: Trodyne (Teterboro, N.J., 1970), Acoustic Emission Technology (Sacramento, Calif., 1972), and Physical Acoustics (Princeton, N.J., 1978) (Drouillard, 1996).
From 1980-1996, decline in heavy industry, decline in the use of nuclear reactors for power generation, and the end of the Cold War and break-up of the Soviet Union (and, thus, less defense spending) slowed AE research and instrumentation development, in some areas. In other areas, however, AE activity grew. Fowler (Monsanto Company, St. Louis, Mo., 1984) developed an AE inspection program that virtually eliminated catastrophic failures in fiber reinforced plastic vessels and piping. A number of AE studies related to tool wear and cutting processes, machine monitoring, bearing friction, and friction in rotating members were reported. Deterioration of many concrete bridges around the world led to increased AE research related to monitoring concrete and civil structures. According to Drouillard, in 1996, the U.S. Department of Transportation rated one in three U.S. bridges either structurally deficient (unable to support standard loads) or functionally obsolete. Seismology studies, for earthquake detection and mine failure prediction, mirrored studies of AE in metals and other materials, since rocks and metals under stress produce similar acoustic emission events. During the period, research related to AE in wood and AE methods for inspecting wood and wood products for defects, cracks, or pests also increased (Drouillard, 1996).
From 1980-1996, several new AE sensor or signal processing techniques were developed. Rockwell International and the National Bureau of Standards developed capacitive transducers to detect burst-type AE. (Drouillard, 1996). At the Virginia Polytechnic Institute, Stiffler and Henneke (1983) designed and tested AE sensors made from polyvinylidene fluoride (PVDF), a piezoelectric polymer which was originally discovered by Kawai (1969). Other investigators introduced digitizers and computers to capture, process, and analyze AE signals (Drouillard, 1996).
3. Recent AE Sensors
During the time period since Drouillard's review (1996-2002), studies related to developing new AE sensor technologies have focused primarily on new piezoelectric sensor materials or new sensor geometries.
Spedding (1996) explored the effects of geometry on thin-film polyvinylidene fluoride (PVDF) AE sensors. He showed that sensor geometry significantly affects sensor frequency response. His results show that thin-film PVDF could be used for developing frequency-selective, directionally sensitive, or programmable sensors.
Imai et al. (1997) developed a compact thin-film AE sensor for detecting head-disk interaction in magnetic disk devices. They used micromachining techniques to embed the sensor inside the slider, by sputtering a zinc oxide (ZnO) piezoelectric layer between two electrodes formed by electron beam deposition. Using both simulations and experiments, they showed that an embedded thin-film sensor is a viable alternative to conventional lead zirconate titanate (PZT) piezoceramic AE sensors for detecting head-disk interaction.
Brown et al. (1999) developed a piezoelectric sensor using a copolymer composed of polyvinylidene fluoride and triflouroethylene (PVDF/PTrFE). They experimentally compared their sensor to a commercial PZT piezoceramic sensor and a laser interferometer using pencil lead fracture and helium jet AE tests. They also evaluated their sensor, in field tests, for detecting fluid and mechanically generated acoustic emission in a centrifugal pump and a turbocharged diesel engine. Brown et al. concluded that a copolymer piezoelectric AE sensor can be used as a viable alternative to piezoceramic sensors or laser interferometers, while providing a more broad-band and detailed response than resonant piezoceramic sensors.
Sundaresan et al. (2002) developed a continuous or distributed AE sensor composed of PZT piezoceramic fibers suspended in a flexible epoxy matrix. They showed that using a continuous sensor could be more efficient for monitoring large areas on complex structures (e.g., for in-flight aircraft structural health monitoring) than using several conventional AE sensors; a continuous sensor is easier to mount and only requires a single data collection channel for a single sensor strip.
Schoess and Zook (1998) introduced a fundamentally new contact AE sensing technique. They used Honeywell's resonant microbeam MEMS sensor to experimentally show that the frequency of a resonating microbeam mounted in a silicon substrate varies in response to simulated AE events (ultrasonic pulsers and pencil lead fracture tests) applied to the silicon substrate. They believe the sensor shows promise for structural and machinery diagnostic applications.
4. AE Sensors for Tool Condition Monitoring
Most early research studies related to tool condition monitoring during machining operations used piezoceramic or piezoelectric sensors for acoustic emission detection (McBride et al., 1993). Piezoceramic sensors produce a dynamic voltage in response to stress waves, when a static electric field is applied to the sensor material. Piezoelectric sensors use materials which are naturally polarized (or which are polarized during manufacturing processes) to reduce the magnitude of the static electric field needed to bias the sensor (Swanson, 2000).
For tool-wear monitoring, placing a piezoceramic or piezoelectric sensor in contact with the workpiece or cutting tool causes stress (sound) waves traveling through the workpiece or tool to also travel through the sensor. As a result, the sensor generates electrical voltages proportional to the stress waves.
The piezoceramic or piezoelectric transducer must have good acoustic coupling to the source of AE. Often, however, mounting a piezoceramic or piezoelectric sensor in contact with the workpiece interferes with machining operations. As a result, the sensor often needs to be placed in contact with parts of the machining bed or tool holder, which attenuates the AE signals, modifies their spectral and temporal properties, and introduces noise from other sources in the machine (e.g., spindle bearing noise and slideway movement) (McBride et al., 1993).
Piezoceramic materials also give rise to strong mechanical resonances, which results in high sensitivity in only a few narrow frequency bands within the spectrum of interest. Although sensors with narrow sensitivity bands effectively eliminate low-frequency machine noise, they also may miss significant AE events outside their limited sensitivity ranges. The precise electromechanical properties of piezoceramic and piezoelectric transducers also vary from unit to unit. As a result, users must individually calibrate devices. Even with calibration, results strongly depend on the quality of the mechanical coupling between sensor and sensed surface, which, in practice, varies greatly (McBride et al., 1993).
Due to the limitations of piezoceramic and piezoelectric AE sensors, McBride, et al., (1993) developed a technique for detecting tool wear based on laser interferometry. They used a laser light source and fiber optics to produce a miniature and robust probe for detecting acoustic emission by measuring the small amplitude (˜0.1 nm), high frequency (0.1-1 MHz) surface vibrations produced during machining operations. They also demonstrated the technique for probing both the workpiece and the rotating tool holder during face milling of mild steel.
Barton and Reuben (1996) also used laser interferometry to measure both acoustic emission (AE) and surface finish (SF) for monitoring tool insert condition. Their AE and SF sensors used an optical fiber to connect a probe head near the workpiece to optical and detection systems remote from the machining center.
AE sensors for tool condition monitoring, based upon laser interferometry, place a small fiber optic probe approximately 20 mm from a plane end face of the workpiece. Laser light sent through the fiber optic cable reflects off of the face of the workpiece. Reflected light interferes with incoming light, based upon distance from the probe to the workpiece face. Signal processing techniques can be used to determine the instantaneous distance from the probe to the workpiece, based upon measured interference patterns, and, thus, determine characteristics of the stress (sound) waves traveling through the workpiece. Laser interferometer measurements can detect target displacements of less than 1 nm (Barton & Reuben, 1996).
Interferometry offers a highly sensitive method for measuring displacement or vibration that can achieve more accurate acoustic emission measurements than contacting piezoelectric transducers located on the machine bed. Interferometry can also be used for accurate non-contact surface finish (SF) measurement in machining operations (Barton & Reuben, 1996).
According to Barton and Reuben (1996), the greatest advantage of optical AE and SF measurement techniques are that they are non-contacting, with no mechanical loading of the test surface. As a result, optical AE measurement techniques guarantee reproducibility of coupling between the transducer and the measured surface. Optical SF measurement techniques also eliminate the possibility of damaging soft surfaces during measurement. In addition, output signals from interferometry are directly traceable to the light wavelength used and are, therefore, absolutely calibrated.
Limitations of AE sensors using laser interferometry include: high cost, small distance required between probe and workpiece, limitations of workpiece geometry (for proper laser light reflection), appearance and routing of the fiber-optic cable, interference with tool movement, and signal contamination due to coolant fluids and other contaminants
In more recent tool condition monitoring research, investigators have pursued a multi-sensor approach, combining piezoceramic, piezoelectric, or optical AE sensors with other types of sensors, to improve tool condition monitoring accuracy over single-sensor methods.
Chi and Dornfield (1998) combined both piezoceramic acoustic emission and cutting force sensors, with an expert system using decision trees and group method data handling, to improve tool wear estimation and prediction accuracy (to within 5% of measured values) over a model created using stepwise regression analysis.
Quan, et al. (1998) also used a multi-sensor approach, by combining an acoustic emission sensor and a Hall-effect power sensor, to detect tool wear with 96% accuracy and calculate actual tool wear with 90% accuracy. They concluded that using a multi-sensor approach, with a neural network to evaluate the multi-sensor data, improves tool wear detection accuracy over a single-sensor method under complex and changing machining conditions.
In 2002, Sick reviewed 138 prior publications related to tool wear monitoring in turning operations. At that time, Sick still rated even the most promising tool wear monitoring methods not marketable due to lack of precision and insufficient generalization capability (operation restricted to a single machine tool, to a specific combination of work material and tool coating, or a small range of cutting conditions).
From Sick's (2002) review, tool wear monitoring remains a difficult problem yet to be solved. From the research to date, machining processes have been classified as non-linear time-variant systems, which are difficult to model. In addition, sensor limitations have made machining processes difficult to measure.
Prior AE studies, particularly studies related to tool condition monitoring in machining operations, have led to two primary types of AE sensors: piezoceramic or piezoelectric crystals and laser interferometry systems.
For tool condition monitoring, piezoceramic and piezoelectric sensors exhibit severe limitations. To operate properly, they must be in contact with an object through which the acoustic waves are traveling. Investigators cannot practically install sensors on the tool or workpiece. Thus, they usually place sensors on the machining bed or tool holder. Sensor placement away from the workpiece leads to signal attenuation, modified spectral and temporal properties, and added noise from other sources in the machine (e.g., spindle bearing noise and slideway movement).
Piezoceramic materials also give rise to strong mechanical resonances, which can result in adequate sensor sensitivity in only a few narrow frequency bands within the spectrum of interest. The precise electromechanical properties of such transducers also vary from unit to unit, so that users must individually calibrate sensors. Even with calibration, results depend strongly on the quality of the mechanical coupling between transducer and surface, which, in practice, varies greatly.
AE sensors based upon laser interferometry also suffer from severe limitations. They require a small distance between sensor probe and workpiece. As a result, users may find it difficult to precisely mount the sensor probe and route the sensor's fiber-optic cable. In addition, the sensor and cable may interfere with tool movement during machining operations and detract from machine appearance. Laser interferometry-based sensors also suffer from signal contamination due to coolant fluids and other optical contaminants. In addition, laser systems are typically expensive.
Therefore, it should be appreciated that the prior art discloses numerous examples of AE sensors which have problems or disadvantages in particular applications.
Although seemingly unrelated without having the benefit of the present disclosure, the use of microwave Doppler radar detectors are now discussed. A microwave Doppler radar detector is designed to “sense” object motion using the Doppler shift phenomenon. The detector emits a high-frequency electromagnetic signal. If the signal reflects off of an object moving toward or away from the sensor, the transmitted signal increases or decreases in frequency, with respect to the original transmitted signal frequency. A receiver in the Doppler radar detector captures the reflected signal, compares the transmitted and received frequencies, and produces an IF (intermediate frequency) output signal with frequency proportional to the velocity of the moving object. IF output signal amplitude varies as a complex function of the size and reflectivity of the sensed object and the object's distance from the sensor (Microwave Solutions, 2002).
Microwave Doppler radar has been used in many practical applications, such as law enforcement systems for detecting automobile speed and commercial systems for opening doors as customers approach department stores. However, to the inventor's knowledge, microwave Doppler radar has never been used to directly detect acoustic emission.
Surface acoustic wave (SAW) devices, based upon microwave radar technology, have been used, for many years, to create many different non-contact wireless sensors (Bulst et al., 2001):                1. Temperature sensors (radio-requestable clinical thermometers; temperature sensors on rotating turbine blades, train brakes, centrifuges, and tires; temperature sensors in hot, dangerous, or inaccessible process chambers)        2. Pressure sensors        3. Torque sensors (tap drill torque alarm)        4. Current sensors        5. Chemical sensors        6. Humidity sensors        7. Mechatronic applications        
The SAW approach uses a microwave radar signal to interrogate a sensor composed of a microwave antenna etched on a material substrate which responds to a given physical condition in a known manner (Steindel, et al., 1999). The antenna converts the interrogating radar signal into a surface acoustic wave which propagates along the substrate and reflects off of metal strips etched on the surface of the substrate at known distances from the antenna. The acoustic wave reflections return to the antenna and transform back into microwave signals which then transmit substrate condition information to a remote receiver.
As the SAW sensor substrate contracts, elongates, or bends in response to surrounding physical conditions, returned signal characteristics change. Users can then determine physical conditions affecting the substrate by processing and analyzing returned signal characteristics. Using different substrate materials creates sensors which can be used to measure different physical environmental properties.
Tyren (2001), in an international patent application (WO 01/73389 A1), describes a non-contact method for measuring mechanical properties such as torque, force, or pressure. The described method uses a vibrating string element to sense changes in the measured quantity and a microwave interrogating signal. Variations in the measured quantity change the tension in the vibrating string, and thus change the vibration frequency of the string element. Changes in vibration frequency of the string element change the amplitude of the microwave interrogating signal.
According to the patent application, “a microwave signal can be amplitude modulated by a mechanically oscillating object in the signal path between a transmitter and receiver” (Tyren, 2001, p. 1). The patent application also describes experiments conducted to verify the phenomenon, using an approximately 10 cm long guitar string segment, in oscillation at 150 Hz, to amplitude modulate a 1.3 GHz microwave signal.
By changing the oscillation frequency of the guitar string to 230 Hz, the inventor measured a change in microwave signal amplitude modulation that followed the change in frequency.
The patent application further describes several possible applications for the phenomenon, including a method for measuring the torque in a rotating axle. The method uses a string element placed along a main tension line of the axle. A mounting mechanism holds the string away from the surface of the axle, so that the string can oscillate freely. Natural system vibrations, or a clapper, induce string vibration. As torque in the axle changes, the oscillation frequency of the string also changes. Again, a high-frequency electromagnetic signal source radiates the vibrating string. Variations in the amplitude of the reflected high-frequency signal indicate variations in torque within the rotating axle. Tyren reportedly used a microwave Doppler radar detector and a band-pass filter tuned to the vibration frequency of a metal guitar string to detect changes in torque due to cutting operations on a manual lathe.
In an earlier study, the inventor worked with Tyren to evaluate his sensor design for detecting changes in torque due to cutting operations on a CNC lathe. Tyren supplied a hand-made microwave radar detector and a band-pass filter needed to detect a vibrating guitar string. The hand-made microwave radar system did not work. As a result, Tyren recommended using an MDU 1620 Motion Detector Unit from Microwave Solutions (http://www.microwave-solutions.com) as a microwave radar source and described the characteristics of a band-pass filter needed to detect a vibrating guitar string. The inventor designed a circuit composed of the Microwave Solutions Motion Detector Unit and a band-pass filter to meet Tyren's specifications for detecting a vibrating guitar string. In testing, the circuit did not appear to work well for detecting torque changes due to cutting operations on a CNC lathe, because workpiece and lathe spindle motion created radar detector signals that were much greater in magnitude than the signals created by a vibrating guitar string attached to the rotating workpiece.
Therefore, it is a primary object, feature, or advantage of the present invention to improve upon the state of the art.
It is a further object, feature, or advantage of the present invention to overcome the limitations of current piezoceramic, piezoelectric and laser-interferometry AE sensors.
Another object, feature, or advantage of the present invention is to provide a method and apparatus for an AE sensor which is economical, yet accurate.
Yet another object, feature, or advantage of the present invention is to provide an AE sensor appropriate for tool condition monitoring in machining operations as well as other applications.
These and/or other objects, features, or advantages of the present invention will become apparent from the specification and claims that follow.